use super::ty::{AllowPlus, RecoverQPath};
use super::{Parser, TokenType};
use crate::maybe_whole;
use rustc_ast::ast::{
self, AngleBracketedArgs, Ident, ParenthesizedArgs, Path, PathSegment, QSelf,
};
use rustc_ast::ast::{
AnonConst, AssocTyConstraint, AssocTyConstraintKind, BlockCheckMode, GenericArg,
};
use rustc_ast::token::{self, Token};
use rustc_errors::{pluralize, Applicability, PResult};
use rustc_span::source_map::{BytePos, Span};
use rustc_span::symbol::{kw, sym};
use log::debug;
use std::mem;
/// Specifies how to parse a path.
#[derive(Copy, Clone, PartialEq)]
pub enum PathStyle {
/// In some contexts, notably in expressions, paths with generic arguments are ambiguous
/// with something else. For example, in expressions `segment < ....` can be interpreted
/// as a comparison and `segment ( ....` can be interpreted as a function call.
/// In all such contexts the non-path interpretation is preferred by default for practical
/// reasons, but the path interpretation can be forced by the disambiguator `::`, e.g.
/// `x<y>` - comparisons, `x::<y>` - unambiguously a path.
Expr,
/// In other contexts, notably in types, no ambiguity exists and paths can be written
/// without the disambiguator, e.g., `x<y>` - unambiguously a path.
/// Paths with disambiguators are still accepted, `x::<Y>` - unambiguously a path too.
Type,
/// A path with generic arguments disallowed, e.g., `foo::bar::Baz`, used in imports,
/// visibilities or attributes.
/// Technically, this variant is unnecessary and e.g., `Expr` can be used instead
/// (paths in "mod" contexts have to be checked later for absence of generic arguments
/// anyway, due to macros), but it is used to avoid weird suggestions about expected
/// tokens when something goes wrong.
Mod,
}
impl<'a> Parser<'a> {
/// Parses a qualified path.
/// Assumes that the leading `<` has been parsed already.
///
/// `qualified_path = <type [as trait_ref]>::path`
///
/// # Examples
/// `<T>::default`
/// `<T as U>::a`
/// `<T as U>::F::a<S>` (without disambiguator)
/// `<T as U>::F::a::<S>` (with disambiguator)
pub(super) fn parse_qpath(&mut self, style: PathStyle) -> PResult<'a, (QSelf, Path)> {
let lo = self.prev_token.span;
let ty = self.parse_ty()?;
// `path` will contain the prefix of the path up to the `>`,
// if any (e.g., `U` in the `<T as U>::*` examples
// above). `path_span` has the span of that path, or an empty
// span in the case of something like `<T>::Bar`.
let (mut path, path_span);
if self.eat_keyword(kw::As) {
let path_lo = self.token.span;
path = self.parse_path(PathStyle::Type)?;
path_span = path_lo.to(self.prev_token.span);
} else {
path_span = self.token.span.to(self.token.span);
path = ast::Path { segments: Vec::new(), span: path_span };
}
// See doc comment for `unmatched_angle_bracket_count`.
self.expect(&token::Gt)?;
if self.unmatched_angle_bracket_count > 0 {
self.unmatched_angle_bracket_count -= 1;
debug!("parse_qpath: (decrement) count={:?}", self.unmatched_angle_bracket_count);
}
if !self.recover_colon_before_qpath_proj() {
self.expect(&token::ModSep)?;
}
let qself = QSelf { ty, path_span, position: path.segments.len() };
self.parse_path_segments(&mut path.segments, style)?;
Ok((qself, Path { segments: path.segments, span: lo.to(self.prev_token.span) }))
}
/// Recover from an invalid single colon, when the user likely meant a qualified path.
/// We avoid emitting this if not followed by an identifier, as our assumption that the user
/// intended this to be a qualified path may not be correct.
///
/// ```ignore (diagnostics)
/// <Bar as Baz<T>>:Qux
/// ^ help: use double colon
/// ```
fn recover_colon_before_qpath_proj(&mut self) -> bool {
if self.token.kind != token::Colon
|| self.look_ahead(1, |t| !t.is_ident() || t.is_reserved_ident())
{
return false;
}
self.bump(); // colon
self.diagnostic()
.struct_span_err(
self.prev_token.span,
"found single colon before projection in qualified path",
)
.span_suggestion(
self.prev_token.span,
"use double colon",
"::".to_string(),
Applicability::MachineApplicable,
)
.emit();
true
}
/// Parses simple paths.
///
/// `path = [::] segment+`
/// `segment = ident | ident[::]<args> | ident[::](args) [-> type]`
///
/// # Examples
/// `a::b::C<D>` (without disambiguator)
/// `a::b::C::<D>` (with disambiguator)
/// `Fn(Args)` (without disambiguator)
/// `Fn::(Args)` (with disambiguator)
pub fn parse_path(&mut self, style: PathStyle) -> PResult<'a, Path> {
maybe_whole!(self, NtPath, |path| {
if style == PathStyle::Mod && path.segments.iter().any(|segment| segment.args.is_some())
{
self.struct_span_err(path.span, "unexpected generic arguments in path").emit();
}
path
});
let lo = self.token.span;
let mut segments = Vec::new();
let mod_sep_ctxt = self.token.span.ctxt();
if self.eat(&token::ModSep) {
segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
}
self.parse_path_segments(&mut segments, style)?;
Ok(Path { segments, span: lo.to(self.prev_token.span) })
}
pub(super) fn parse_path_segments(
&mut self,
segments: &mut Vec<PathSegment>,
style: PathStyle,
) -> PResult<'a, ()> {
loop {
let segment = self.parse_path_segment(style)?;
if style == PathStyle::Expr {
// In order to check for trailing angle brackets, we must have finished
// recursing (`parse_path_segment` can indirectly call this function),
// that is, the next token must be the highlighted part of the below example:
//
// `Foo::<Bar as Baz<T>>::Qux`
// ^ here
//
// As opposed to the below highlight (if we had only finished the first
// recursion):
//
// `Foo::<Bar as Baz<T>>::Qux`
// ^ here
//
// `PathStyle::Expr` is only provided at the root invocation and never in
// `parse_path_segment` to recurse and therefore can be checked to maintain
// this invariant.
self.check_trailing_angle_brackets(&segment, token::ModSep);
}
segments.push(segment);
if self.is_import_coupler() || !self.eat(&token::ModSep) {
return Ok(());
}
}
}
pub(super) fn parse_path_segment(&mut self, style: PathStyle) -> PResult<'a, PathSegment> {
let ident = self.parse_path_segment_ident()?;
let is_args_start = |token: &Token| match token.kind {
token::Lt
| token::BinOp(token::Shl)
| token::OpenDelim(token::Paren)
| token::LArrow => true,
_ => false,
};
let check_args_start = |this: &mut Self| {
this.expected_tokens.extend_from_slice(&[
TokenType::Token(token::Lt),
TokenType::Token(token::OpenDelim(token::Paren)),
]);
is_args_start(&this.token)
};
Ok(
if style == PathStyle::Type && check_args_start(self)
|| style != PathStyle::Mod
&& self.check(&token::ModSep)
&& self.look_ahead(1, |t| is_args_start(t))
{
// We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
// it isn't, then we reset the unmatched angle bracket count as we're about to start
// parsing a new path.
if style == PathStyle::Expr {
self.unmatched_angle_bracket_count = 0;
self.max_angle_bracket_count = 0;
}
// Generic arguments are found - `<`, `(`, `::<` or `::(`.
self.eat(&token::ModSep);
let lo = self.token.span;
let args = if self.eat_lt() {
// `<'a, T, A = U>`
let (args, constraints) =
self.parse_generic_args_with_leading_angle_bracket_recovery(style, lo)?;
self.expect_gt()?;
let span = lo.to(self.prev_token.span);
AngleBracketedArgs { args, constraints, span }.into()
} else {
// `(T, U) -> R`
let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
let span = ident.span.to(self.prev_token.span);
let output = self.parse_ret_ty(AllowPlus::No, RecoverQPath::No)?;
ParenthesizedArgs { inputs, output, span }.into()
};
PathSegment { ident, args, id: ast::DUMMY_NODE_ID }
} else {
// Generic arguments are not found.
PathSegment::from_ident(ident)
},
)
}
pub(super) fn parse_path_segment_ident(&mut self) -> PResult<'a, Ident> {
match self.token.ident() {
Some((ident, false)) if ident.is_path_segment_keyword() => {
self.bump();
Ok(ident)
}
_ => self.parse_ident(),
}
}
/// Parses generic args (within a path segment) with recovery for extra leading angle brackets.
/// For the purposes of understanding the parsing logic of generic arguments, this function
/// can be thought of being the same as just calling `self.parse_generic_args()` if the source
/// had the correct amount of leading angle brackets.
///
/// ```ignore (diagnostics)
/// bar::<<<<T as Foo>::Output>();
/// ^^ help: remove extra angle brackets
/// ```
fn parse_generic_args_with_leading_angle_bracket_recovery(
&mut self,
style: PathStyle,
lo: Span,
) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
// We need to detect whether there are extra leading left angle brackets and produce an
// appropriate error and suggestion. This cannot be implemented by looking ahead at
// upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
// then there won't be matching `>` tokens to find.
//
// To explain how this detection works, consider the following example:
//
// ```ignore (diagnostics)
// bar::<<<<T as Foo>::Output>();
// ^^ help: remove extra angle brackets
// ```
//
// Parsing of the left angle brackets starts in this function. We start by parsing the
// `<` token (incrementing the counter of unmatched angle brackets on `Parser` via
// `eat_lt`):
//
// *Upcoming tokens:* `<<<<T as Foo>::Output>;`
// *Unmatched count:* 1
// *`parse_path_segment` calls deep:* 0
//
// This has the effect of recursing as this function is called if a `<` character
// is found within the expected generic arguments:
//
// *Upcoming tokens:* `<<<T as Foo>::Output>;`
// *Unmatched count:* 2
// *`parse_path_segment` calls deep:* 1
//
// Eventually we will have recursed until having consumed all of the `<` tokens and
// this will be reflected in the count:
//
// *Upcoming tokens:* `T as Foo>::Output>;`
// *Unmatched count:* 4
// `parse_path_segment` calls deep:* 3
//
// The parser will continue until reaching the first `>` - this will decrement the
// unmatched angle bracket count and return to the parent invocation of this function
// having succeeded in parsing:
//
// *Upcoming tokens:* `::Output>;`
// *Unmatched count:* 3
// *`parse_path_segment` calls deep:* 2
//
// This will continue until the next `>` character which will also return successfully
// to the parent invocation of this function and decrement the count:
//
// *Upcoming tokens:* `;`
// *Unmatched count:* 2
// *`parse_path_segment` calls deep:* 1
//
// At this point, this function will expect to find another matching `>` character but
// won't be able to and will return an error. This will continue all the way up the
// call stack until the first invocation:
//
// *Upcoming tokens:* `;`
// *Unmatched count:* 2
// *`parse_path_segment` calls deep:* 0
//
// In doing this, we have managed to work out how many unmatched leading left angle
// brackets there are, but we cannot recover as the unmatched angle brackets have
// already been consumed. To remedy this, we keep a snapshot of the parser state
// before we do the above. We can then inspect whether we ended up with a parsing error
// and unmatched left angle brackets and if so, restore the parser state before we
// consumed any `<` characters to emit an error and consume the erroneous tokens to
// recover by attempting to parse again.
//
// In practice, the recursion of this function is indirect and there will be other
// locations that consume some `<` characters - as long as we update the count when
// this happens, it isn't an issue.
let is_first_invocation = style == PathStyle::Expr;
// Take a snapshot before attempting to parse - we can restore this later.
let snapshot = if is_first_invocation { Some(self.clone()) } else { None };
debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
match self.parse_generic_args() {
Ok(value) => Ok(value),
Err(ref mut e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
// Cancel error from being unable to find `>`. We know the error
// must have been this due to a non-zero unmatched angle bracket
// count.
e.cancel();
// Swap `self` with our backup of the parser state before attempting to parse
// generic arguments.
let snapshot = mem::replace(self, snapshot.unwrap());
debug!(
"parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
snapshot.count={:?}",
snapshot.unmatched_angle_bracket_count,
);
// Eat the unmatched angle brackets.
for _ in 0..snapshot.unmatched_angle_bracket_count {
self.eat_lt();
}
// Make a span over ${unmatched angle bracket count} characters.
let span = lo.with_hi(lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count));
self.struct_span_err(
span,
&format!(
"unmatched angle bracket{}",
pluralize!(snapshot.unmatched_angle_bracket_count)
),
)
.span_suggestion(
span,
&format!(
"remove extra angle bracket{}",
pluralize!(snapshot.unmatched_angle_bracket_count)
),
String::new(),
Applicability::MachineApplicable,
)
.emit();
// Try again without unmatched angle bracket characters.
self.parse_generic_args()
}
Err(e) => Err(e),
}
}
/// Parses (possibly empty) list of lifetime and type arguments and associated type bindings,
/// possibly including trailing comma.
fn parse_generic_args(&mut self) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
let mut args = Vec::new();
let mut constraints = Vec::new();
let mut misplaced_assoc_ty_constraints: Vec<Span> = Vec::new();
let mut assoc_ty_constraints: Vec<Span> = Vec::new();
let args_lo = self.token.span;
loop {
if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
// Parse lifetime argument.
args.push(GenericArg::Lifetime(self.expect_lifetime()));
misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
} else if self.check_ident()
&& self.look_ahead(1, |t| t == &token::Eq || t == &token::Colon)
{
// Parse associated type constraint.
let lo = self.token.span;
let ident = self.parse_ident()?;
let kind = if self.eat(&token::Eq) {
AssocTyConstraintKind::Equality { ty: self.parse_ty()? }
} else if self.eat(&token::Colon) {
AssocTyConstraintKind::Bound {
bounds: self.parse_generic_bounds(Some(self.prev_token.span))?,
}
} else {
unreachable!();
};
let span = lo.to(self.prev_token.span);
// Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
if let AssocTyConstraintKind::Bound { .. } = kind {
self.sess.gated_spans.gate(sym::associated_type_bounds, span);
}
constraints.push(AssocTyConstraint { id: ast::DUMMY_NODE_ID, ident, kind, span });
assoc_ty_constraints.push(span);
} else if self.check_const_arg() {
// Parse const argument.
let expr = if let token::OpenDelim(token::Brace) = self.token.kind {
self.parse_block_expr(
None,
self.token.span,
BlockCheckMode::Default,
ast::AttrVec::new(),
)?
} else if self.token.is_ident() {
// FIXME(const_generics): to distinguish between idents for types and consts,
// we should introduce a GenericArg::Ident in the AST and distinguish when
// lowering to the HIR. For now, idents for const args are not permitted.
if self.token.is_bool_lit() {
self.parse_literal_maybe_minus()?
} else {
let span = self.token.span;
let msg = "identifiers may currently not be used for const generics";
self.struct_span_err(span, msg).emit();
let block = self.mk_block_err(span);
self.mk_expr(span, ast::ExprKind::Block(block, None), ast::AttrVec::new())
}
} else {
self.parse_literal_maybe_minus()?
};
let value = AnonConst { id: ast::DUMMY_NODE_ID, value: expr };
args.push(GenericArg::Const(value));
misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
} else if self.check_type() {
// Parse type argument.
args.push(GenericArg::Type(self.parse_ty()?));
misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
} else {
break;
}
if !self.eat(&token::Comma) {
break;
}
}
// FIXME: we would like to report this in ast_validation instead, but we currently do not
// preserve ordering of generic parameters with respect to associated type binding, so we
// lose that information after parsing.
if !misplaced_assoc_ty_constraints.is_empty() {
let mut err = self.struct_span_err(
args_lo.to(self.prev_token.span),
"associated type bindings must be declared after generic parameters",
);
for span in misplaced_assoc_ty_constraints {
err.span_label(
span,
"this associated type binding should be moved after the generic parameters",
);
}
err.emit();
}
Ok((args, constraints))
}
}